1. Field of the Invention
The present invention relates to resonant power converters, and particularly relates to a control circuits for resonant power converters.
2. Background of the Invention
For achieving higher efficiency, a resonant power converter should operate at a frequency closely to the resonant frequency of the resonant power converter with a full load and/or with a low input voltage. However, when the switching frequency decreases in response to increase of the load and/or decrease of the input voltage of the power converter, the switching frequency might fall into a zero current switching (ZCS) region (that is, region 3 shown in FIG. 2). If the switching frequency decreases and falls into region 3, the resonant power converter would operate at a none-linear area. Besides, the none-ZVS (non-zero voltage switching) operation will cause overheat in switching power devices and generate noises. Therefore, preventing the resonant power converter from operating in region 3 in FIG. 2 is required.